An ultrasonic flowmeter emits an ultrasonic wave into a fluid, receives the emitted ultrasonic wave to obtain a flow velocity, and converts the obtained flow velocity into a flow rate of a fluid to measure the flow rate (Non-Patent Document 1). An ultrasonic sensor which is a piezoelectric vibrator is used to emit or receive the ultrasonic wave. As a method of obtaining the flow velocity, there is a method using a Doppler effect, and the like. However, a method for measuring a transfer time difference, which uses ultrasonic sensors disposed at an upstream side and a downstream side of a pipe, respectively, and includes: obtaining a flow velocity based on a difference between a propagation time of an ultrasonic wave transmitted to the upstream side and a propagation time of an ultrasonic wave transmitted to the downstream side; and calculating the flow rate by the obtained flow velocity, has been widely used.
The method for measuring the transfer time difference uses a gate configured to measure time based on an ultrasonic wave transmitting timing and an ultrasonic wave receiving timing between the ultrasonic sensors disposed at the upstream and downstream sides of the pipe, and a high speed counter to measure an ultrasonic propagation time to the upstream side and an ultrasonic propagation time to the downstream side. As a timing detecting method, there is a zero-crossing method to measure a position at which the received ultrasonic signal is zero-crossed.
Meanwhile, a correlation method obtains the propagation time to the upstream side and the propagation time to the downstream side based on an autocorrelation peak time of a transmission waveform and a reception waveform.
The ultrasonic flowmeter has also been used in equipments such as a boiler to measure a flow rate of a fluid under high temperature and high pressure conditions. When an outlet temperature of the boiler is about 100° C., a sensor using piezoelectric zirconate titanate (PZT) which is piezoelectric ceramic has been mainly used in the related art. However, a Curie point of the PZT is about 150 to 250° C. depending on a composition thereof, and a piezoelectric constant thereof is remarkably reduced in the vicinity of the Curie point. For this reason, in order to measure a flow rate of a fluid in a region exceeding 200° C., a sensor using a piezoelectric material or a piezoelectric single crystal material having a higher Curie point than the PZT has been used (Non-Patent Document 2).
As the method of ultrasonic flow measurement in a high temperature region, a method using a sensor of a conventional PZT based material by cooling the sensor has been proposed. For example, Patent Document 1 discloses a method for obtaining a flow rate of a high temperature fluid according to a transfer time difference principle by disposing a pipe through which the high temperature fluid flows within a container filled with a low temperature liquid and disposing ultrasonic sensors on pipe walls of the upstream and downstream sides of the pipe so that the ultrasonic sensors are cooled by the low temperature liquid. Further, Patent Document 2 discloses a configuration to prevent a temperature of the piezoelectric vibrator from increasing due to heat from the high temperature fluid by installing a sound transmission passage of a quartz material between the piezoelectric vibrator and the high temperature fluid at the time of measuring the flow rate of the high temperature fluid.
Meanwhile, as a piezoelectric material, there is lithium niobate (LiNbO3: hereinafter briefly referred to as an LN) which has a much higher Curie point than the PZT and may withstand high temperature conditions. General properties of the LN are described in Non-Patent Document 5. The Curie point of the LN is about 1200° C. FIG. 1(a) illustrates a crystal structure of the LN. The LN has a crystal structure of a trigonal system and as illustrated in FIG. 1, an X-axis, a Y-axis, and a Z-axis are crystallographically defined. Further, as a lattice constant of the LN, a=b=5.148 Å and c=13.863 Å.
When the LN is used as the ultrasonic sensor of the ultrasonic flowmeter, an ultrasonic wave having a short duration needs to be generated as a burst wave and thus vibration needs to be dumped. In order to dump the vibration, a metal piece (that is, dumper) is attached to the LN vibrator. As the attachment position of the metal piece, there are two cases, that is, a case in which the metal piece is attached on the same surface as an output surface of the ultrasonic wave in the ultrasonic sensor as described in Patent Document 4, or the like, and a case in which the metal piece is attached on a surface opposite to the output surface of the ultrasonic wave as described in Patent Document 5, or the like. Patent Document 4 discloses that an aluminum alloy lead material is used for bonding the dumping portion to the piezoelectric vibrator, and Patent Document 5 discloses that silver (Ag) is used as the dumping portion for bonding the dumping portion to the piezoelectric vibrator by eutectic bonding between thin films of silver and gold (Au). Further, Patent Document 7 discloses that a metal shoe which forms a temperature gradient while serving as the dumping portion is bonded to the piezoelectric vibrator made of a ferroelectric material having a high Curie point. In addition, Patent Document 10 discloses that as a lead material for performing the bonding to the LN piezoelectric vibrator, an Al—Si—Mg alloy or a silver solder is used and as the silver solder, a material containing 45% Ag, 16% Cu, 24% Cd and the remainder being Zn is used. Further, Patent Document 10 discloses that when the LN piezoelectric vibrator is bonded to a protective layer made of a cermet insulating material, a thin film of Cu or Ni is formed on a surface of the cermet insulating material by an ion plating, a silver electrode is formed on the piezoelectric vibrator, and then the silver electrode of the piezoelectric vibrator is bonded to the cermet insulating material by the silver solder.
When a single crystal of the LN which is the trigonal system is thermally expanded, anisotropy is present in a coefficient of linear expansion, and even though the coefficient of linear expansion in an X-axis direction and the coefficient of linear expansion in a Y-axis direction are the same, the coefficient of linear expansion in a Z-axis direction is different therefrom. Considering that the metallic dumping portion is bonded to the piezoelectric vibrator made of the LN single crystal, when the dumping portion is bonded to a surface other than a surface (so called “Z cut surface”) orthogonal to the Z-axis in the LN, the anisotropy occurs within the bonded surface during the thermal expansion, and therefore cracks may be generated in the piezoelectric vibrator due to a heat cycle applied thereto, and the like. However, as described in Non-Patent Document 3 and the like, a piezoelectric coefficient in the Z-axis direction in the LN single crystal is smaller than that of other general piezoelectric materials. For this reason, the ultrasonic sensor, in which the LN piezoelectric vibrator is not damaged even if the heat cycle is applied thereto, has reduced transmission or reception capabilities of the ultrasonic wave and does not accurately measure the flow rate. Table 1 shows characteristics such as the Curie point, the piezoelectric coefficient, and a relative dielectric constant, in various piezoelectric materials, and Table 2 shows a coefficient thermal expansion (coefficient of linear expansion) in the LN or other materials. In the Table 1, a Z cut plate represents an LN plate cut along two parallel Z cut surfaces and a Y 36° cut plate represents the LN plate cut along two paralell Y-axis 36° cut surfaces to be described below.
TABLE 1LiNbO3LiNbO3Piezoelectric(Y 36° cut(Z cutMaterialplate)plate)PbNb2O6PbTiO3PZTCurie Point (° C.)11501150530385150 to295Piezoelectric4068044470CoefficientD33 (pC/N)Relative39293001851500 toDielectric3000Constant εDensity (g/cm3)4.464.465.77.67.65Sound73403800—45004600Velocity (m/s)
TABLE 2Coefficient of Linear Expansion at 25 toMaterial850° C. (×10−6 K−1)LiNbO3 (X-axis direction, 5.15 to 2.25Y-axis direction)LiNbO3 (Z-axis direction)13.85 to 3.88Silver18.9 or moreStainless Steel (SUS304)14.8Pure Titanium 8.4 or moreFrit glass (SiO2—B2O3—ZnO) 7.65
When the flowmeter is configured using the ultrasonic wave, for example, it is necessary for the ultrasonic sensor to be mechanically and acoustically bonded to the pipe or a spool piece installed on the pipe. In this case, the ultrasonic wave from the ultrasonic sensor needs to be efficiently transferred to the pipe, the spool piece, or the like, and therefore a couplant (contact medium) is applied to a contact portion of the pipe, the spool piece, or the like. When the flowmeter for high temperature is manufactured, as the couplant, a material withstanding high temperature is used. For example, Patent Documents 6 to 8 disclose a couplant which includes water glass as a main ingredient and has appropriate flexibility or viscosity in a measurement temperature region. After the ultrasonic sensor is manufactured, the couplant including the water glass as a main ingredient is disposed to the ultrasonic sensor by application, or the like. Patent Document 9 also discloses that an electrode made of a heat resistant soft metal having appropriate plasticity in the measurement temperature region and the electrode is used as the couplant. Further, as the couplant for high temperature, examples using a gold foil or a copper foil and silver have been known in the related art.
The ultrasonic flowmeter is used a principle of obtaining a flow velocity based on, for example, the difference between the ultrasonic transfer time in a flow direction and the ultrasonic transfer time in a direction opposite to the flow direction, and measuring the flow rate by the obtained flow velocity (Non-Patent Document 1). Therefore, it is preferable that the ultrasonic flowmeter has a small Q value for the ultrasonic signal and small reverberation as a whole of the ultrasonic flowmeter. When the zero-crossing method or the correlation measurement method is used for measuring the transfer time difference, it is important to reduce, in particular, the reverberation.
To reduce the reverberation, for example, a thin protective film or a retarder is disposed on a front surface of an ultrasonic probe for nondestructive inspection or medical treatment. The retarder also serves as the above-described dumping portion. When an acoustic impedance (herein, referred to as an “intrinsic acoustic impedance” represented by a product of the sound velocity and the density of the material) of the retarder is close to the acoustic impedance of the piezoelectric vibrator, the ultrasonic wave generated from the piezoelectric vibrator is transferred to the retarder and vibration energy disappears by being scattered in the vibrator. Therefore, multiple reflections, that is, resonances are rapidly damped within the vibrator as much. As the acoustic impedance of the vibrator is close to that of the retarder, the vibration energy inside the piezoelectric vibrator is transferred to an outside thereof, and therefore the Q value of the vibrator is reduced and an output waveform thereof becomes a waveform having a small ringing. However, a piezoelectric vibrator having a high Q value has been used in the related art. Patent Document 3 discloses that the reverberation appears to be small by using a propagation auxiliary member in a two vibrator type ultrasonic probe used for nondestructive inspection, or the like.